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This chapter is from the book

This chapter is from the book

Extending a Lease and Moving Between Subnets

Using the configuration file in Example 3.11, let's take a closer look at some of the examples from Chapter 2. The section "Using DHCP to Configure Computers" describes the steps in configuring a GSI client, desktop1, when it is first connected to the 192.168.12.0 subnet of the GSI network. When the DHCP server, dhcpserve, receives the initial broadcast message from desktop1, the server determines that desktop1 is connected to the 192.168.12.0 subnet. From the configuration file, the DHCP server determines that desktop1 should receive an address in the range 192.168.12.1 to 192.168.12.253. To accompany that selected address, the DHCP server selects the subnet-specific values for the subnet mask and default router, and it selects global values for the domain name and domain name servers. Because desktop1 did not request a specific lease time, the DHCP server chooses a lease time of 30 days, and the server returns the parameters in Table 3.2 to desktop1. At the same time, the server records the information about the allocated address and lease time on disk.

Table 3.2 Initial Parameters for desktop1

Option Name

Option Value

IP address

192.168.12.25

Subnet mask

255.255.255.0

Default router

192.168.12.254

Domain name

genericstartup.com

Domain name servers

dns1.genericstartup.com, dns2.genericstartup.com

Lease time

30 days (2,592,000 seconds)


Extending a GSI Lease

The section "Restarting desktop1" in Chapter 2 describes the sequence of events that occur when desktop1 restarts while it is still connected to the 192.168.12.0 subnet. In that situation, desktop1 broadcasts a DHCP message to confirm its address, 192.168.12.25. The DHCP server on dhcpserve receives the confirmation request, and it consults its configuration and lease data. Based on the entry for the 192.168.12.0 subnet, the server confirms that 192.168.12.25 is a valid address for the network segment to which desktop1 is currently connected. The server then consults the lease data and confirms that desktop1 has a valid lease on 192.168.12.25. Having established the validity of the requested address, the DHCP server extends the lease on the address to the default value of 30 days and returns an acknowledgment. After receiving the acknowledgment, desktop1 records the new lease time and begins to use its old address again.

Moving Between GSI Subnets

Suppose, now, that desktop1 is re-located to a new subnet, as described in the section "Moving desktop1 to a New Network Segment" in Chapter 2. As in the previous example, desktop1 first broadcasts a message to confirm its address. But in this case, the DHCP server determines that 192.168.12.25 is not a valid address for the segment to which it is attached. The server examines the address recorded in the DHCP message by the relay agent, 192.168.13.254, and identifies the 192.168.13.0 subnet as the source of the message. Because 192.168.12.25 is not in the range of addresses on that subnet, the server returns a message denying desktop1 the use of the requested address.

After receiving the negative reply from the DHCP server, desktop1 restarts the DHCP process as it would if it had no valid address. It broadcasts an initial message to locate dhcpserve, which allocates an IP address, 192.168.13.37, from the range of addresses available on the new network to which desktop1 is now attached. desktop1 records the new address and begins using the network with its newly assigned address.

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